<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .main {
            width: 100px;
            height: 100px;
            background: #abcdef;
            display: flex;
            align-items: center;
            justify-content: center;
        }
    </style>
</head>

<body>
    <div class="main">嘿嘿嘿</div>
    <button>click</button>
</body>
<script>
    const btn = document.querySelector('button')
    const dom = document.querySelector('div')
    btn.onclick = () => {
        dom.style.position = 'fixed'
        dom.style.left = '0'
        dom.style.top = '0'
        dom.style.width = '100vw'
        dom.style.height = '100vh'
        dom.style.zIndex = '999'
    }
</script>

</html>